Yes, monsters now are actually very tame. But back in Season 1, there were quite a few that were meant to be scary. A lot of them were holdovers from when the Muppets appeared on variety shows Ed Sullivan, Jack Paar, etc. Jim designed them, Don Sahlin built them, and Frank performed them. They were pretty ugly, had sharp teeth, and were not meant to be friendly. They were MONSTERS, and they were INTENDED to be scary. That was the idea. And the idea worked all too well.
Back in 1969, there was no Elmo, no Zoe, no Telly, no Rosita, no monster that was remotely cute or cuddly. Grover back then was an ugly brownish-green, and Cookie used to interrupt Kermit's lectures, eat the props, and once tried to eat Kermit (very first episode, Kermit demonstrating the letter W).
Like I said, back in the first season some Muppet skits were pretty edgy, and the monsters were supposed to make you a little nervous. Maybe you had to be there, says the old geezer...